1. Field of the Invention
This invention relates to copolymerizable blocked isocyanate diols useful in urethane condensation polymerization reactions. The invention also relates to a process for the preparation of such blocked isocyanate diols, and in a further aspect is directed to crosslinkable, linear polyurethanes of high molecular weight prepared employing the blocked isocyanate diols.
2. Description of the Prior Art
It is well known that condensation of an organic diisocyanate such as toluene diisocyanate with an organic polyol such as polypropylene glycol in the presence, as may be necessary, of other additives results in polyurethanes useful for various purposes.
In many fields of application, such as in manufacturing polyurethane films, foams, textiles, shaped articles, coating compounds and wire enamels, a one-package urethane composition is desired which is stable at room temperature, but reacts at elevated temperatures as if free isocyanate groups were present to form the desired polyurethane. Compositions of this type contain "blocked" isocyanates which are prepared by reacting an organic isocyanate with a blocking agent, which contains an active hydrogen atom, such as a phenol, an aliphatic or aromatic amine, etc. The isocyanate compound thus blocked has only limited thermal stability, generally dissociating at temperatures of 150.degree. to 200.degree. C., to regenerate the free isocyanate group, which can react further with, e.g., a polymer containing active hydrogen atoms to form an insoluble polymer. For example, U.S. Pat. No. 2,568,885 discloses the preparation of non-crosslinkable polyurethanes of high molecular weight by polymerizing blocked diisocyanates with difunctional compounds containing active hydrogens such as diols, diamines, aminoalcohols, and dicarboxylic acids. U.S. Pat. No. 3,833,525 teaches a one-package polyurethane system consisting of blocked diisocyanate, a polyol and a nitrogen-containing heterocyclic compound, in which system the isocyanate groups are liberated at a temperature as low as 70.degree. C.
Blocked isocyanate groups may be used not only in preparing polyurethanes, but they may also be present in the final polymer to provide a means for crosslinking the polymer (see Saunders and Frisch, "Polyurethanes: Chemistry and Technology", Vol. XVI, Part II, New York: Interscience Publishers, 1962, pp. 489-490). Typically, crosslinkable, terminally blocked isocyanate polymers are prepared by reacting a polyisocyanate with a polyol or a hydroxy-terminated polymer to a low molecular weight using an excess of polyisocyanate to form an isocyanate terminated prepolymer, which is then reacted with a blocking agent.
The crosslinkable, terminally blocked polymers of the prior art are generally of low molecular weight, because if they were polymerized to a high molecular weight, they would contain very few terminally blocked isocyanate groups, thereby limiting their potential for crosslinking. Because of their low molecular weight, these polymers cannot be molded or shaped into useful films or articles before being crosslinked, so that their applicability is quite limited.
Japanese Pat. No. 70 34,708 discloses blocked isocyanate monomers containing an ethylenically unsaturated group which are polymerized to form thermosetting polyurethane resins of high molecular weight.
The present invention provides a blocked isocyanate diol which will copolymerize to form a linear, crosslinkable polyurethane of high molecular weight containing relatively many blocked isocyanate groups per polymer chain, which can be shaped into useful articles before being crosslinked.